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States ready for gay marriage battles

The Commonwealth is throwing down the gauntlet to the ACT on gay marriage as legal experts in Tasmania say there's no reason states shouldn't be able to make their own laws on the issue.

Commonwealth Attorney-General George Brandis has confirmed the federal government will challenge the ACT's same-sex marriage law in the High Court once it passes, which could happen within the next four weeks.

"Irrespective of anyone's views on the desirability or otherwise of same-sex marriage, it is clearly in Australia's interests that there be nationally consistent marriage laws," Senator Brandis said.

The Commonwealth Marriage Act provided that consistency but the ACT's law would be "a threat to that well-established position", he said.

ACT Attorney-General Simon Corbell said he and Senator Brandis had a "polite but forthright" discussion about the matter during a meeting of the standing committee on law and justice in Sydney on Thursday.

"We will be robustly defending our law and asserting that our law is capable of concurrent operation with the Commonwealth law and that it is not inconsistent," he told AAP.

A Tasmania Law Reform Institute report released on Thursday has found no legal reason for states not to make laws on marriage.

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But its authors do say there is no way to predict how the High Court would rule on a challenge.

The island state made a bid to go it alone on gay marriage last year when its lower house became the first in the country to pass legislation on the issue.

It was narrowly defeated in the upper house because of concerns about its legitimacy under the constitution and whether it left Tasmania open to a costly High Court challenge.

In the ACT, the first same-sex weddings could happen as early as December.

Marriage Equality chair and independent NSW MP Alex Greenwich said the fact the federal government was intervening would encourage same-sex couples to get married sooner rather than later.

"The more people we have expressing their love and commitment will make it harder for any laws to be overturned," he told AAP.

But Senator Brandis warned it might be distressing for same-sex couples who marry under the new law, only to have their union later invalidated by a High Court challenge.

"It would be better for all concerned if the ACT government waited for a short time until the validity of the proposed law was determined by the High Court," he said.

But Mr Corbell said the territory had "declined to do that" because there's strong support for the law, which could pass within the next four weeks.

"We are disappointed that the Commonwealth professes concern for same-sex couples entering into marriage in case the law is struck down when it is they themselves who are seeking to have it struck down," he told AAP.